It is known that at gas pressures in a laser cavity exceeding a few tens of torr the discharges periodically generated between a pair of excitation electrodes may become irregular, changing from a uniform glow to one or more localized arcs to the detriment of the emitted beams. This drawback can be eliminated by the use of ancillary corona electrodes which pre-ionize the gaseous atmosphere of the cavity. In some instances, separate high-voltage pulse sources are being used for the energization of the main and the ancillary electrodes; these sources might include, for example, diferent trigger switches in the form of a spark gap and a thyratron, respectively. It is also known to energize the corona electrodes in parallel with the main electrodes by the same high-voltage spikes through a common spark gap; see, for instance, U.S. Pat. No. 4,292,600.